On Tue, Oct 26, 2021 at 06:42:03PM +0000, SeongJae Park wrote: > Hello Changbin, > > On Tue, 26 Oct 2021 23:30:33 +0800 Changbin Du <changbin.du@xxxxxxxxx> wrote: > > > An kernel thread can exit gracefully with kthread_stop(). So we don't need a > > new flag 'kdamond_stop'. And to make sure the task struct is not freed when > > accessing it, get task struct on start and put it on stop. > > We previously considered using kthread_stop() here. However, we resulted in > current code because kdamond can be self-terminated when all target processes > are invalid[1]. > > Seems this patch is also not fully prepared for the self-termination case. I > left some comments below. > > [1] https://lore.kernel.org/linux-mm/20210624102623.24563-1-sjpark@xxxxxxxxx/ > > > > > And since the return value of 'before_terminate' callback is never used, > > we make it have no return value. > > This looks nice to me. Could you please send this again as a separate patch? > Sure, I'll do it later. > > > > Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx> > > --- > > include/linux/damon.h | 3 +-- > > mm/damon/core.c | 59 +++++++++++++------------------------------ > > mm/damon/dbgfs.c | 5 ++-- > > 3 files changed, 20 insertions(+), 47 deletions(-) > > > > diff --git a/include/linux/damon.h b/include/linux/damon.h > > index a14b3cc54cab..041966786270 100644 > > --- a/include/linux/damon.h > > +++ b/include/linux/damon.h > [...] > > --- a/mm/damon/core.c > > +++ b/mm/damon/core.c > [...] > > @@ -1069,7 +1048,7 @@ static int kdamond_fn(void *data) > > sz_limit); > > if (ctx->callback.after_aggregation && > > ctx->callback.after_aggregation(ctx)) > > - set_kdamond_stop(ctx); > > + done = true; > > kdamond_apply_schemes(ctx); > > kdamond_reset_aggregated(ctx); > > kdamond_split_regions(ctx); > > @@ -1088,16 +1067,12 @@ static int kdamond_fn(void *data) > > damon_destroy_region(r, t); > > } > > > > - if (ctx->callback.before_terminate && > > - ctx->callback.before_terminate(ctx)) > > - set_kdamond_stop(ctx); > > + if (ctx->callback.before_terminate) > > + ctx->callback.before_terminate(ctx); > > if (ctx->primitive.cleanup) > > ctx->primitive.cleanup(ctx); > > > > pr_debug("kdamond (%d) finishes\n", current->pid); > > - mutex_lock(&ctx->kdamond_lock); > > - ctx->kdamond = NULL; > > - mutex_unlock(&ctx->kdamond_lock); > > When kdamond is self-terminating, ctx->kdamond will not be nullfified. As a > result, this patch can introduce some errors like below: > > # cd /sys/kernel/debug/damon > # sleep 60 & > [1] 1926 > # echo $(pidof sleep) > target_ids > # echo on > monitor_on > # cat monitor_on > on > # # after 60 seconds, sleep finishes and kdamond is self-terminated > # cat monitor_on > off > # echo 42 > target_ids > bash: echo: write error: Device or resource busy > > If we simply restore the nullification here with the mutex locking, we would > result in a deadlock because __damon_stop() calls kthread_stop() while holding > ctx->kdamond_lock. > > Also, the reference count of ctx->kdamond, which increased by __damon_start(), > would not be decreased in the case. > If so, I suppose below change should work correctly (still set ctx->kdamond to NULL at the end of kdamond). static int __damon_stop(struct damon_ctx *ctx) { + struct task_struct *tsk; + mutex_lock(&ctx->kdamond_lock); - if (ctx->kdamond) { - ctx->kdamond_stop = true; + tsk = ctx->kdamond; + if (tsk) { + get_task_struct(tsk); mutex_unlock(&ctx->kdamond_lock); - while (damon_kdamond_running(ctx)) - usleep_range(ctx->sample_interval, - ctx->sample_interval * 2); + kthread_stop(tsk); + put_task_struct(tsk); return 0; } mutex_unlock(&ctx->kdamond_lock); > If I'm missing something, please let me know. > > > Thanks, > SJ > > [...] -- Cheers, Changbin Du
